CrimpJiggler
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Understanding the underlying principles of organic chemistry
Lately I've been trying to figure out why mechanisms are the way they are by thinking about the electron flow and it seems to be working. For example
Markovnikovs rule for hydrohalogenations of alkenes. Alkyl groups are inductive donators. The alkenyl carbon with more alkyl substituents will have
more electrons to spare because its alkyl substituents inductively donate charge to it. Consequently, the other less substituted alkenyl carbon will
gain more of the double bond charge because the other alkenyl carbon doesn't need it as much. Therefore, the electrophile will form a sigma bond with
the less alkyl substituted carbon simply because it has more of the pi charge. I'm guessing that Markovnikovs rule only applies to alkenes with
electron donating substituents. For example, in the case of 1,1-difluoroethylene, I bet the electrophile will add to the carbon with the F
substituents instead. When I first started learning organic chem, I read about how tertiary carbocations are the most stable while primary
carbocations are the least stable and how its the opposite for carbanions. Again, alkyl groups are inductive donors so I see now why they stabilise
carbocations: by pushing negative charge towards the positive charge and partially neutralizing it.
I learned this concept of inductive/resonance donors/acceptors when I was learning about EAS reactions but since then I've learned how it applies to
acids, bases, leaving groups, nucleophiles, electrophiles etc. I still get very confused by intramolecular electron transfer and various other aspects
of organic mechanisms but I think I'm making progress. Have any of you got to the point where you can figure out the mechanism to pretty much any
common reaction scheme you come across simply by understanding how the electron flow works?
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francis
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Quote: Originally posted by CrimpJiggler  | | Lately I've been trying to figure out why mechanisms are the way they are by thinking about the electron flow... the electrophile will form a sigma
bond with the less alkyl substituted carbon simply because it has more of the pi charge. |
Hi CrimpJiggler,
Yep its the right sort of idea you have, but you can extend your reasoning by considering not just electronegativity, and lone pairs, but atomic
orbitals, and molecular orbitals.
(CH3)3-C=CH-(CH3)2 is more stabilised at the tertiary carbon not just due to inductive effects, because also because of the stability of the
carbocation: the tertiary centre has nine C-H sigma orbitals, each of which can overlap with the empty p orbital of the carbocation in a process
called ‘hyperconjugation’ (the methyl groups are not planar, they are tetrahedral so they cannot fully conjugate with the trigonal carbocation).
The secondary centre only has six C-H sigma orbitals available to participate in hyperconjugation.
The C-H sigma orbital at the secondary carbon will necessarily be orthogonal to an empty p-orbital so cannot stabilise it (they can’t overlap at all
with the p orbital).
| Quote: Originally posted by CrimpJiggler | | I'm guessing that Markovnikovs rule only applies to alkenes with electron donating substituents. For example, in the case of 1,1-difluoroethylene, I
bet the electrophile will add to the carbon with the F substituents instead. |
The electrophile will add to the difluoro-substituted carbon, but not for the reasons you thought.
Markovnikov’s rule is just another way of looking at Hammond’s postulate: since the activation energy of the reaction pathway leading to the
Markovnikov carbocation is lower in energy than that leading to the anti-Markovnikov carbocation, the Markovnikov carbocation forms faster and this
reaction pathway is followed.
Think of the carbocation: the C-F sigma bonds adjacent to the charge can stabilise the charge by delocalisation (even though it is highly
electronegative, a nonbonding fluorine orbital overlaps with the empty carbocation p orbital to form a bonding MO, and the electrons from fluorine
will go into the bonding MO which is overall lower in energy than the two atomic orbitals used to form it).
No such stabilisation exists if the charge is on the other carbon. (Hyperconjugation can’t occur, since a carbocation is planar, so the carbocation
is planar, the C-H sigma orbital can’t overlap with the p orbital).
| Quote: Originally posted by CrimpJiggler | | I still get very confused by intramolecular electron transfer and various other aspects of organic mechanisms but I think I'm making progress. Have
any of you got to the point where you can figure out the mechanism to pretty much any common reaction scheme you come across simply by understanding
how the electron flow works? |
I certainly can’t figure out the mechanism to any reaction by understanding electron flow: when you start to learn organic chemistry you get taught
valence bond theory, but it’s hard to rationalise, say, pericyclic reactions without considering molecular orbitals.
For me, writing mechanisms has always been tough. I try to look at as many important facets of the chemistry as I can before writing a mechanism. I
used to write them quickly (and often incorrectly). Writing reactions for transition metal reactions like the Heck reaction etc I have found more
difficult.
Lots of reactions have no agreed-upon mechanism at all, and new reactions are examined by physical organic chemists to determine how the overall
transformation occurs.
I would recommend two books for understanding mechanisms: Peter Sykes Guidebook to Mechanism in Organic Chemistry, and Robert Grossman’s The Art of
Writing Reasonable Organic Reaction Mechanisms.
[Edited on 3-12-2011 by francis]
[Edited on 3-12-2011 by francis]
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CrimpJiggler
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Thanks francis. I just read up on hyperconjugation. Interesting stuff. Hyperconjugation probably explains why alkyl substituents stabilise radicals
better than inductive donation does. In fact, inductive donation alone would probably destabilise a radical wouldn't it. Then again I read that
radicals can be stabilised by allowing the unpaired electron to combine with an electron pair to form a 3 electron pi bond. That is, 2 electrons in
the bonding orbital, 1 in the antibonding orbital.
As for Markovnikovs rule, good point, I wasn't taking vicinal carbocation stability into consideration. While the fluoro substituted carbon would be
more electron rich, the fluorine atoms would destabilise the vicinal carbocation. That was a highly informative reply, thanks.
[Edited on 4-12-2011 by CrimpJiggler]
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